mark as uncompatible with python <3.8

[nyanloutre]

Hello,

When installing the latest tag with python 3.6 I am getting the following error :

    gcc -pthread -Wno-unused-result -Wsign-compare -DDYNAMIC_ANNOTATIONS_ENABLED=1 -DNDEBUG -O2 -g -pipe -Wall -Werror=format-security -Wp,-D_FORTIFY_SOURCE=2 -Wp,-D_GLIBCXX_ASSERTIONS -fexceptions -fstack-protector-strong -grecord-gcc-switches -m64 -mtune=generic -fasynchronous-unwind-tables -fstack-clash-protection -fcf-protection -D_GNU_SOURCE -fPIC -fwrapv -O2 -g -pipe -Wall -Werror=format-security -Wp,-D_FORTIFY_SOURCE=2 -Wp,-D_GLIBCXX_ASSERTIONS -fexceptions -fstack-protector-strong -grecord-gcc-switches -m64 -mtune=generic -fasynchronous-unwind-tables -fstack-clash-protection -fcf-protection -D_GNU_SOURCE -fPIC -fwrapv -O2 -g -pipe -Wall -Werror=format-security -Wp,-D_FORTIFY_SOURCE=2 -Wp,-D_GLIBCXX_ASSERTIONS -fexceptions -fstack-protector-strong -grecord-gcc-switches -m64 -mtune=generic -fasynchronous-unwind-tables -fstack-clash-protection -fcf-protection -D_GNU_SOURCE -fPIC -fwrapv -fPIC -I/tmp/tmp.7nSTVd4avl/venv/include -I/usr/include/python3.6m -c pystemd/journal.c -o build/temp.linux-x86_64-3.6/pystemd/journal.o
    pystemd/journal.c:31:6: error: #error Cython requires Python 3.8+.
         #error Cython requires Python 3.8+.
          ^~~~~
    error: command 'gcc' failed with exit status 1

I identified the change as a Cython update in your CI pipeline. C files are now generated using cython 3 since 0.13.4 making it incompatible with python 3.6 and 3.7.

If you want to maintain compatibility a solution is to force cython in version <3 during CI.

Also version 0.13.4 should be removed from pypi as it is breaking pip install with python 3.6

journal.c in 0.13.2:

/* Generated by Cython 0.29.35 */

/* BEGIN: Cython Metadata
{
    "distutils": {
        "depends": [],
        "libraries": [
            "systemd"
        ],
        "name": "pystemd.journal",
        "sources": [
            "pystemd/journal.pyx"
        ]
    },
    "module_name": "pystemd.journal"
}
END: Cython Metadata */
[...]

journal.c in 0.13.4:

/* Generated by Cython 3.1.2 */

/* BEGIN: Cython Metadata
{
    "distutils": {
        "depends": [],
        "libraries": [
            "systemd"
        ],
        "name": "pystemd.journal",
        "sources": [
            "pystemd/journal.pyx"
        ]
    },
    "module_name": "pystemd.journal"
}
END: Cython Metadata */
[...]